Chromium plating is an electrochemical process that is well-known in the art. There are two general types of chromium plating, hard chromium plating and decorative chromium plating. Hard chromium plating involves the application of a heavy coating of chromium onto steel substrates, typically to prevent wear, and exists in thicknesses in the range of about 10 to about 1000 μm. Decorative chromium plating applies a much thinner layer of chromium, in the range of about 0.25 to about 1.0 μm, and provides an extremely thin but hard coating for aesthetic purposes to achieve a shiny, reflective surface and/or protect against tarnish, corrosion and scratching of the metal beneath.
For decorative purposes, the chromium is generally applied over a coating of nickel. The chromium provides a hard, wear-resistant layer and excellent corrosion performance is obtained due to the chromium layer being cathodic with respect to the underlying nickel deposit. The underlying nickel layer becomes the anode in the corrosion cell and corrodes preferentially, leaving the chromium layer uncorroded.
Decorative chromium has traditionally been electroplated from electrolytes containing hexavalent chromium using, for example, an aqueous chromic acid bath prepared from chromic oxide (CrO3) and sulfuric acid. However, many attempts have been made to develop a commercially acceptable process for electroplating chromium using electrolytes containing only trivalent chromium ions. The incentive to use electrolytes containing trivalent chromium salts arises because hexavalent chromium presents serious health and environmental hazards. Hexavalent chromium ion and its solutions have technical limitations including the ever-increasing cost of disposing of plating baths and rinse water. Furthermore, the operation of plating from baths containing substantially hexavalent chromium ion has operational limits which increase the probability of producing commercially unacceptable deposits.
For many years, chromium has been electro-deposited from electrolytes containing chromic acid using lead anodes. Lead anodes are commonly used because the cathodic efficiency of the process is quite low (usually no higher than 25%) so the use of soluble chromium anodes is not possible because it would cause a build-up of chromium metal in the plating bath. A secondary function of the lead anodes is to re-oxidize trivalent chromium produced in the plating bath at the cathode which is achieved via the formation of a lead dioxide coating at the surface of the anodes during electrolysis. In these baths, the chromium metal can simply be replaced by adding more chromic acid.
Because of the toxicity of chromic acid, chromium plating electrolytes based on trivalent chromium have more recently been developed. While these baths are safer to use than hexavalent baths, they rely on dragout of the plating solution in order to keep the solution in balance. Techniques such as drag-out recovery or partial “closed loop” techniques cannot be used with these processes because the source of chromium metal in the bath is a chromium salt (typically chromium sulfate). As the chromium is plated out of the bath, more chromium sulfate has to be added, resulting in a build-up of sulfate in the bath which can lead to problems if drag-out recovery or “closed loop” systems are employed.
Re. 35,730 to Reynolds, the subject matter of which is herein incorporated by reference in its entirety, describes a process and apparatus for regenerating a plating bath comprising trivalent chromium cations with an ion exchange resin, preferably a cation exchange resin to selectively remove impurities from the plating bath. The ion exchange column is connected to the plating tank. However, this system requires the use and disposal of ion exchange resins.
Therefore, it would be advantageous if the chromium metal in trivalent electroplating baths could be replenished by electrolytic dissolution of chromium metal in order to maintain the metal content of the bath. While this may appear to be a matter of simply applying an anodic potential to chromium metal anodes, in fact this is not possible in practice. The reason for this is that chromium is a very active metal that readily forms an oxide layer on its surface, which renders the chromium passive. Upon applying an anodic potential to this passive chromium, little dissolution of chromium occurs until the potential becomes sufficiently anodic as to exceed the trans-passive potential. At this point, the current increases and the chromium begins to dissolve. However, at the highly anodic potential required for this step, the chromium dissolves as hexavalent chromium, which is a severe poison for trivalent chromium electrolytes and will prevent the electrolyte from working. Consequently, there is no known method for dissolving chromium electrolytically continuously for a chromium metal electrode as trivalent chromium.
Passive chromium can be activated by making it cathodic and liberating hydrogen at the surface. Unfortunately, it re-passivates very quickly. Surprisingly, the inventors of the present invention have found that by applying an alternating series of cathodic and anodic current “pulses” to a chromium electrode that chromium dissolves readily from the chromium metal electrode in the form of trivalent chromium. The present invention has many potential applications for maintaining chromium metal content in processes containing trivalent chromium, including, for example, chromium plating and chromium passivation processes.